Hi
I'm not a newbie to synth building so I may be a little embarrased here..

I have just got hold of a bunch of 2206 chips after purchasing a copy of Thomas' VCO Chip Cookbook.

I have tried breadboarding both of the simplest VCO setups for this chip. Currently I have the "standard arrangement" set up. All I am getting however is a lot of distorted noise... vaguely tuneable distorted noise mind! Obviously something is wrong.

Tuning the 1M variable resistor I get howling noise over most of the range, but a small section of the range with a cleaner very high pitched tone. The amplitude trimmer seems to have no effect on the level at all.

Could I have bought a bunch of dodgy chips?

I have checked and double checked everything... and as I say I have built plenty of complex circuits before so I am a little puzzled.

I have tried several from the same batch. Maybe I'm just being an idiot and missing something.

It is a shame thought that all the VCO chips in Thomas's book are discontinued - it's a great book and highly educational. The only place to buy any of these chips is eBay which doesn't give me the same confidence as Farnell etc.

I've run into a similar problem with a bunch of NOS 2206 chips I bought. One thing you might try is running the chip on a lower supply voltage. Try 11 or even 10 volts. I found that did the trick, although it does mess with the output levels. Something tells me that these chips we're seeing don't work as well with higher current levels. I'm experimenting with changes to the El Cerrito (keeping the PCB the same, just changing a few Rs) to see if I can come up with a fix for these chips, so we'll see. Also, in the US, Jameco sells the 2206 for about $6. I have a few of these that I'm going to try to see if they behave okay at 15V.

Just finished one using the Bug Brand layout - with a 15V supply i'm getting harsh noise on the output. On 12V it all seems fine except that I can hear a rather prominent crackling noise on frequencies between 500Hz and 3kHz - I've tried 3 different chips from this batch:

2206CP
F0919

Also on 12V the wave morphing works but doesn't go all the way to a perfect saw shape. I guess this is due to the lower supply voltage...

Hey,
Anyone have any joy with their XR2206 circuits?
I just completed a second Mega Percussive Synth (to be 200% sure that I'd not soldered badly, or messed up the wiring) and it acts exactly like the first - the Shell generator just creates noise rather than the triangle wave.

The chips I'm using are XR2206CP F0919

I've ordered some more from Cricklewood Electronics so will let you know once I've swapped out the chips...

Hey chivaros - I can confirm that those chips are dodgy. I was getting bad results from that batch as well. Yesterday I received a military grade XR2206 in ceramics package, which performs very well in Thomas Henry's XR-VCO.

on a slightly different tack -
if i wanted to build this exponential converter for split-supply and remove the linear FM option - what would be the right approach?
do i just tie q1 and q2's emitters to ground?

It's good to know there are SOME date codes that are okay. I've been through several chips, and the only one that works properly in the "El Cerrito" XR VCO is an F0747 date code. It might also be worth noting that this one has a small stamped indentation at the OPPOSITE end from the Pin 1 notch. All of the ones I have that only work on lower voltages have the stamped indentation at the same end as the Pin 1 notch.

And indeed the +15V single supply I specified is well below (by 11V) the spec sheet claims. So that's certainly not the problem.

Thomas Henry

Dear all,

I recently tested a batch of XR2206 (I do not know the batch number anymore) and came to the conclusion that something has changed
in the design or production process. They also gave a lot of distortion
with the power supply set to +/- 7V5.
The I made a base setup with a single supply and noticed the distortion
disappearing at 11V5.
I then realized that two chips I bought in 2002 (batch 0044 = 2000 week 44) which I at the time wanted to use for the TD Cross Generator were
put aside by me as not good. I rested them with 11V and they were OK.

I looks like that the ones I bought in the early eighties worked fine
with +/- 12V.

Conclusion: If you want to use the Thomas Henry design, I suggest you lower down the supply voltage to 11V.

Note:
Thanks to Thomas for his great work, in particular for the SKEW option, which I now implemented in a modified way to the TD Cross Generator.

I am having a nightmare of a time getting the TH XR-VCO running. I must've etched six+ boards now and ruined at least four+ of them. Have a look at (only some) of the wreckage and cringe:

I am at my wits end with the thing, and so I thought I'd attach a close-up of the last board I soldered in the hopes that someone might be able to tell me (a newbie) exactly what I'm doing wrong.

A few notes in relation to the close-up. On this particular board, the TL074 becomes extremely hot upon power up. (On an earlier board, the TL074 would always just snap like a firecracker!) The tempco is also moved out of the way intentionally here, in order to make the LM394 more visible. In any case, all the resistor/capacitor values have been checked and triple checked, as has the solder underside (for any possible shorts). All of the semiconductors/ICs have been replaced numerous times. I've tried running +15V (red wires bottom left) and -15V (black wires bottom center): no sound out (from orange wire on the top, third from the right, nor from purple wire on the bottom). I've tried running +15V & -15V with all of the DPDT wires properly attached to a DPDT switch, still, no sound out. I've tried hooking up the coarse and skew in addition to everything just mentioned, still no sound. I've tried hooking up ALL of the pots in addition to everything mentioned, still no sound (out of the sine or square outputs).

In short, then, I have been unable to get this thing to make any sound, and I wonder what the hell I'm doing wrong! (Something with the power? The diode orientations? The IC orientations? Etc.) Please don't laugh at me if it is something obvious, I've lost more than few nights sleep trying to get the XR-VCO going. Hopefully someone can tell me what I'm doing wrong, but if not, I should have some El-Cerito boards on the way once Magic Smoke returns from moving. It's just, I figure, if I can't get /this one/ going, how am I supposed to manage anything else! Please let me know... Thanks...

Thanks guys... I just can't tell you how frustrating this has been. What I /wanted/ to do is post all sorts of cool photos of the finished the instrument, not etch six+ boards, solder five of them, and not get a single one of them to work!! I've checked my power supply with TWO multimeters and it reads +15V coming in (black to black / red to red). Now, I'm including a photo of the solder-side:

I know it isn't pretty. But any scratch that is evident in this photo has been tested and it isn't stopping the flow of current. The same goes for the joints, they've all been checked. In other words, given that the same thing has happened now with FOUR+ boards, it has to be that I'm doing something extremely wrong, something unrelated to the PCB, something unrelated to the soldering, but something ABSOLUTELY wrong.

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